Point coverage badge at codecov instead of coveralls

README.md

@@ -30,7 +30,7 @@ Released under the [MIT License](LICENSE.txt).
 [doc]: https://godoc.org/go.uber.org/atomic
 [ci-img]: https://travis-ci.org/uber-go/atomic.svg?branch=master
 [ci]: https://travis-ci.org/uber-go/atomic
-[cov-img]: https://coveralls.io/repos/github/uber-go/atomic/badge.svg?branch=master
-[cov]: https://coveralls.io/github/uber-go/atomic?branch=master
+[cov-img]: https://codecov.io/gh/uber-go/atomic/branch/master/graph/badge.svg
+[cov]: https://codecov.io/gh/uber-go/atomic
 [reportcard-img]: https://goreportcard.com/badge/go.uber.org/atomic
 [reportcard]: https://goreportcard.com/report/go.uber.org/atomic